When can a / function be included in C ++? Can coercive behavior?
I am trying to get the expected behavior when I use a keyword inline
. I tried to call the function in different files, templating the function using different inline function implementation, but no matter what I do, the compiler never inlines the function.
So in which case exactly would the compiler choose a built-in function in C ++?
Here is the code I tried:
inline auto Add(int i) -> int {
return i+1;
}
int main() {
Add(1);
return 0;
}
In this case, I get:
Add(int):
pushq %rbp
movq %rsp, %rbp
movl %edi, -4(%rbp)
movl -4(%rbp), %eax
addl $1, %eax
popq %rbp
ret
main:
pushq %rbp
movq %rsp, %rbp
movl $1, %edi
call Add(int)
movl $0, %eax
popq %rbp
ret
Or again,
template<typename T>
inline auto Add(const T &i) -> decltype(i+1) {
return i+1;
}
int main() {
Add(1);
return 0;
}
And I got:
main:
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
movl $1, -4(%rbp)
leaq -4(%rbp), %rax
movq %rax, %rdi
call decltype ({parm#1}+(1)) Add<int>(int const&)
movl $0, %eax
leave
ret
decltype ({parm#1}+(1)) Add<int>(int const&):
pushq %rbp
movq %rsp, %rbp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movl (%rax), %eax
addl $1, %eax
popq %rbp
ret
I used https://gcc.godbolt.org/ to get the build code here, but I also tried on my machine with clang and gcc (with and without optimization options).
EDIT:
Ok, I am missing something with the optimization options. If I configure GCC to use the o3 optimization layer, my method is inlined .
But still. How does GCC or another compiler know when to inline a function or not?
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Typically, your code is always inlined only if you specify:
__attribute__((always_inline))
for example (from gcc documentation):
inline void foo (const char) __attribute__((always_inline));
Though it's almost never a good idea to force your compiler to inline your code.
You can set the optimization level to a high (although the O flag) to achieve maximum insertion, but see the gcc documentation for more information
Nesting is actually controlled by a number of parameters. You can install them using the -finline- * options. You can see them here
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